1. Field of the Invention
The present invention relates to a technology for converting a Commission Internationale de I'Eclairage (CIE) color to a device color.
2. Description of the Related Art
In PostScript language, it is possible to specify colors for drawing objects (text, image, and graphic) in a document. Specifically, it is not only possible to specify a color format, such as RGB or CMYK, that is dependent on specifications of the input and output devices, but also possible to specify a color format called CIE that is independent of the specification of the input and output devices and uses a color system based on human's perception of colors. When colors are specified in the CIE format, a unit called an interpreter internally converts the specified CIE colors into CMYK color values that are supported by the printer engine and are device dependent. The conversion is controlled by two dictionaries: a first dictionary (color space dictionary (CSD)) that defines conversion of an input CIE color space to XYZ stimulus values (intermediate values) of the CIE 1931 XYZ color space, and a second dictionary (color rendering dictionary (CRD)) that defines conversion from XYZ stimulus values (intermediate values) to CMYK color values specific to an output device. How such a conversion can be performed has been described in detail in “PostScript Language Reference, 3rd edition” from Adobe System Incorporated. Because the conversion processing is complicated and requires a large amount of calculations, it takes more time for color conversion than the processing using colors specified in RGB or CMYK, causing a significant degradation in the printing performance.
To suppress the degradation in the printing performance, conversion calculations are performed beforehand with some color samples of CIE colors thereby calculating CMYK color values of those sample colors. Then, a conversion table is prepared that includes the sample color values, i.e., CIE color values, and their equivalent CMYK color values, and color conversion is performed by using the conversion table. To convert CIE colors that are not in the conversion table, a method of interpolation (e.g., tetrahedral interpolation or triangular prism interpolation) is employed. Although the process of creating the conversion table is time consuming, once the conversion table is created, an approximate conversion of almost any color can be performed in a shorter time.
A typical interpreter assigns/applies a single CRD to a single document or page, so that it is sufficient to create the conversion table only once. Further, the interpreter includes a function for improving the finish of the entire document for which colors are specified in CIE, i.e., a color expansion function of a PostScript (PS) interpreter that assigns/applies a CRD that reproduces optimal rendering intents for each type of drawing objects (text, image, and graphic), thereby improving the finish of the entire document for which colors are specified by CIE. This function is also called as a CRD automatic switching function.
Japanese Patent Laid Open No. 2004-103017 discloses a related technology. This publication discloses to embed a render table for graphics and a render table for images by coupled to each other in a one CRD. Further, means is provided that dynamically determines whether a calculation for color conversion using the CRD is to be performed on graphics or images, and allows access to one of the render tables based on the result of the determination. When such a configuration is employed, it becomes possible to perform the color conversion appropriately for each of two different drawing objects by using one CRD.
The expansion function requires the recreation of the CIE color conversion table every time the type of the drawing object switches. Thus, if the number of drawings of PS data to be processed is large, then there is an increase in the number of processing times and the execution time for the creation of CIE color conversion table, causing a significant degradation in performance.
The technology described in Japanese Patent Laid-Open No. 2004-103017 supports only two types of drawing objects: graphic and image. In other words, it does not support text. Further, color conversion is performed for any one of those drawing objects only by switching between the render tables, each serving as an element in the CRD. Thus, there is a limitation on the level for appropriate color conversion.
The technology described in Japanese Patent Laid-Open No. 2004-103017 is not suitable if the drawing object is of other type, such as text, or when other elements (e.g., MatrixPQR or TransformPQR relating to CIE gamut mapping) of the CRD need to be used to improve the level for appropriate color conversion. In other words, it is necessary to implement a method including defining (preparing) a CRD corresponding to each drawing object, switching a CRD corresponding to each drawing object, and then performing CIE color conversion. Thus, this technology fails to solve the problem of degradation in performance.